1. Field of the Invention
The present invention concerns an optical system of collimation that is designed to be mounted on a helmet worn by an observer, that is, a system designed to form a helmet display unit. Its application is more particularly envisaged in the field of aeronautics.
2. Description of the Prior Art
In military aircraft and combat helicopters, the pilot needs to be presented with piloting and fire control information superimposed on the external landscape. This information role is currently fulfilled by the head-up visor present in all modern military aircraft and in certain helicopters. The drawback of this type of display is that it can show an image only in a limited field which is always centered in the axis of the aircraft. By contrast, the pilot may have to do sightings away from the axis of the aircraft, whence the idea of a display related to the position of the pilot's head. A concept such as this requires, firstly, a system enabling the head position to be detected and, secondly, enabling a light image to be prepared and collimated. These two systems have to be placed on the pilot's helmet, and this fact entails very great constraints, notably on weight.
The following are the essential characteristics that ought to be taken into account in making such systems:
minimal visual interference: the optical system collimating the image should offer the least possible hindrance to the view of the outside environment, and should also prevent the creation of a mask in the field of vision;
safety: since the optical system is close to the eye, it should be seen to it that the eye will not be injured in the event of any impact;
weight: the weight should be minimal. Notably, if the system is designed to be mounted on a military aircraft, it should be as light as possible to prevent fatigue, especially under a heavy load factor.
Furthermore, the collimation system should have performance characteristics generally providing for a wide field, resolution compatible with the eye and high luminosity.
In the SPIE document, Display System Optics, vol. 778 (1987), an article by James E. MELZER and Eric W. LARKIN, "An Integral Approach to Helmet Display System Design", pp. 83 to 88, describes a helmet display system comprising: a generator or source of a synthetic light image to be collimated, a collimator or objective for the collimation of the light radiation corresponding to the synthetic image and a combining optical system or combiner formed by a confocal assembly of two parabolic mirrors, a first mirror reflecting the radiation of the collimated image towards the second mirror, which is partially reflective to reflect this radiation towards the observer and simultaneously enables the transmission of the radiation that comes from the exterior and is included in the field of the system. Furthermore, the helmet display unit includes a number of returning mirrors to deflect the optical path and enable assembly on a helmet. In this prior art assembly, the parabolic mirrors work off axis, which is a cause of aberrations and reduces the quality of the image unless the field of the visor is quite severely restricted.